- Fam. 2.—Goniodorididae. Mantle-border projecting; frontal veil reduced, and often covered by the anterior border of the mantle. Goniodoris, British. Acanthodoris, British. Idalia, British. Ancula, British. Doridunculus. Lamellidoris. Ancylodoris, the only fresh-water Nudibranch, from Lake Baikal.
- Fam. 3.—Heterodorididae. No branchia. Heterodoris.
- Fam. 4.—Dorididae. Mantle oval, covering the head and the greater part of the body; anterior tentacles, ill-developed; branchiae generally retractile. Doris, British. Hexabranchus. Chromodoris.
- Fam. 5.—Doridopsidae. Pharynx suctorial; no radula; branchial rosette on the dorsal surface, above the mantle-border. Doridopsis.
- Fam. 6.—Corambidae. Anus and branchia posterior, below the mantle-border. Corambe.
- Fam. 7.--Phyllidiidae. Pharynx suctorial; branchiae surrounding the body, between the mantle and foot. Phyllidia. Fryeria.
The last three families constitute the sub-tribe Porostomata, characterized by the reduction of the buccal mass, which is modified into a suctorial apparatus.
Tribe 3.—Eolidomorpha (Cladohepatica). The whole of the liver contained in the integuments and tegumentary papillae. Genital duct diaulic; male and female apertures contiguous. The anus is antero-lateral, except in the Proctonotidae, in which it is median. Tegumentary papillae not ramified, and containing cnidosacs with nematocysts.
- Fam. 1.—Eolididae. Dorsal papillae spindle-shaped or club-shaped. Eolis, British. Facelina, British. Tergipes, British. Gonieolis. Cuthona. Embletonia. Galvina. Calma. Hero.
- Fam. 2.—Glaucidae. Body furnished with three pairs of lateral lobes, bearing the tegumentary papillae; foot very narrow; pelagic. Glaucus.
- Fam. 3.—Hedylidae. Body elongated; visceral mass marked off from foot posteriorly; dorsal appendages absent, or reduced to a single pair; spicules in the integument. Hedyle.
- Fam. 4.—Pseudovermidae. Head without tentacles; body elongated; anus on right side. Pseudovermis.
- Fam. 5.—Proctonotidae. Anus posterior, median; anterior tentacles, atrophied; foot broad. Janus, British. Proctonotus, British.
- Fam. 6.—Dotonidae. Bases of the rhinophores surrounded by a sheath; dorsal papillae tuberculated and club-shaped, in a single row on either side of the dorsum; no cnidosacs. Doto, British. Gellina. Heromorpha.
- Fam. 7.—Fionidae. Dorsal papillae with a membranous expansion; male and female apertures at some distance from each other; pelagic. Fiona.
- Fam. 8.—Pleurophyllidae. Anterior tentacles in the form of a digging shield; mantle without appendages, but respiratory papillae beneath the mantle-border. Pleurophyllidia.
- Fam. 9.—Dermatobranchidae. Like the last, but wholly without branchiae. Dermatobranchus.
Tribe 4.—Elysiomorpha. Liver ramifies in integuments and extends into dorsal papillae, but there are no cnidosacs. Genital duct always triaulic, and male and female apertures distant from each other. No mandibles, and radula uniserial. Never more than one pair of tentacles, and these are absent in Alderia and some species of Limapontia.
Fig. 55.—Dorsal and Ventral View of Pleurophyllidia lineata (Otto), one of the Eolidomorph Nudibranchs. (After Keferstein.)
|l,||The lamelliform sub-pallial gills, which (as in Patella) replace the typical Molluscan ctenidium.|
- Fam. 1.—Hermaeidae. Foot narrow; dorsal papillae linear or
fusiform, in several series. Hermaea, British. Stiliger. Alderia, British.
- Fam. 2.—Phyllobranchidae. Foot broad; dorsal papillae flattened and foliaceous. Phyllobranchus. Cyerce.
- Fam. 3.—Plakobranchidae. Body depressed, without dorsal
papillae, but with two very large lateral expansions, with dorsal plications. Plakobranchus.
- Fam. 4.—Elysiidae. Body elongated, with lateral expansions;
tentacles large; foot narrow. Elysia, British. Tridachia.
- Fam. 5.—Limapontiidae. No lateral expansions, and no dorsal papillae; body planariform; anus dorsal, median and posterior. Limapontia, British. Actaeonia, British. Cenia.
Order 2 (of the Euthyneura).—Pulmonata. Euthyneurous Gastropoda, probably derived from ancestral forms similar to the Tectibranchiate Opisthobranchia by adaptation to a terrestrial life. The ctenidium is atrophied, and the edge of the mantle-skirt is fused to the dorsal integument by concrescence, except at one point which forms the aperture of the mantle-chamber, thus converted into a nearly closed sac. Air is admitted to this sac for respiratory and hydrostatic purposes, and it thus becomes a lung. An operculum is present only in Amphibola; a contrast being thus afforded with the operculate pulmonate Streptoneura (Cyclostoma, &c.), which differ in other essential features of structure from the Pulmonata. The Pulmonata are, like the other Euthyneura, hermaphrodite, with elaborately developed copulatory organs and accessory glands. Like other Euthyneura, they have very numerous small denticles on the lingual ribbon. In aquatic Pulmonata the osphradium is retained.
In some Pulmonata (snails) the foot is extended at right angles to the visceral hump, which rises from it in the form of a coil as in Streptoneura; in others the visceral hump is not elevated, but is extended with the foot, and the shell is small or absent (slugs).
Fig. 57.—Ancylus fluviatilis, a patelliform aquatic Pulmonate.
Pulmonata are widely distinguished from a small number of Streptoneura at one time associated with them on account of their mantle-chamber being converted, as in Pulmonata, into a lung, and the ctenidium or branchial plume aborted. The terrestrial Streptoneura (represented in England by the common genus Cyclostoma) have a twisted visceral nerve-loop, an operculum on the foot, a complex rhipidoglossate or taenio-glossate radula, and are of distinct sexes. The Pulmonata have a straight visceral nerve-loop, usually no operculum even in the embryo, and a multidenticulate radula, the teeth being equi-formal; and they are hermaphrodite. Some Pulmonata (Limnaea, &c.) live in fresh waters although breathing air. The remarkable discovery has been made that in deep lakes such Limnaei do not breathe air, but admit water to the lung-sac and live at the bottom. The lung-sac serves undoubtedly as a hydrostatic apparatus in the aquatic Pulmonata, as well as assisting respiration.
The same general range of body-form is shown in Pulmonata as in the Heteropoda and in the Opisthobranchia; at one extreme we have snails with coiled visceral hump, at the other cylindrical or flattened slugs (see fig. 56). Limpet-like forms are also found (fig. 57, Ancylus). The foot is always simple, with its flat crawling surface extending from end to end, but in the embryo Limnaea it shows a bilobed character, which leads on to the condition characteristic of Pteropoda.